Power management concept in dc distributed systems

ABSTRACT

A method includes the steps of providing a common DC bus to interconnect power elements to a DC distribution system using power converters. A first group of one or more of the elements (main element) is used to execute the primary function by automatically maintaining the DC bus voltage following a set point. The DC bus voltage set point is intentionally changed with slow dynamics according to a secondary function executed by the main element such that the average DC bus voltage regulated by the main element changes. A local logic is used on each of the power elements connected to the DC bus but different from the main element to modify their power generation or consumption as a result of changes in the measured average DC bus voltage such that they contribute to the fulfillment of the secondary level of control.

CROSS-REFERENCE TO CO-PENDING APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 14/858,856, filed Sep. 18, 2015, which application was acontinuation-in-part of U.S. patent application Ser. No. 13/828,290filed Mar. 14, 2013 and claims priority on U.S. Provisional PatentApplication Ser. No. 62/182,788, filed Jun. 22, 2015.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention is a method and system to interconnect and operateseveral conventional or alternative power generators, loads, and/orenergy storage elements physically separated from each other in a DCdistribution system.

2. Prior Art

DC power distribution systems are used to exchange power among multiplesources (mainly renewable energy resources), loads, and energy storageelements by connecting those elements to a common DC bus using powerconverters or direct connection in a concept generally called DCmicrogrid. A DC microgrid could be as simple as solar+storageinstallation or as complex as multi MW systems with many types ofdifferent generators, multiple ratings and varieties of loads, severaldifferent types of energy storage elements and multiple invertersconnected or separated from the grid.

The reliability of a DC microgrid depends on the control of the DC busvoltage within specified limits, which depends on balancing the powerproduction with the power consumption. Because renewable energy sourcesare intermittent in nature and loads can suddenly change, fast responsefrom power converters and capacitive energy storage in the DC busprovide the means to instantaneously maintain the DC bus. Methods tocontrol the DC bus voltage are well known and developed where one ormultiple elements connected to the DC bus change their power quickly asthe DC bus voltage drifts from the set point to balance the total powerin the bus and maintain the DC bus voltage within acceptable limits.

In power distributed systems where the different elements are distantfrom each other, the regulation of the DC voltage is challenging. Someimplementations use a single element to regulate the DC bus voltage butthe regulating element has to change depending on the operationconditions. In other implementations, the responsibility to regulate theDC bus is shared amongst multiple elements that have to operate incoordination. The droop method where a virtual resistance is introducedat the output of the power converters that participate of the DC busvoltage regulation has been successfully used in practical applications.However, physical distances between elements used in the DC bus voltageregulation compromise the stability and reliability of the system. Inaddition to maintaining the DC bus voltage, in most cases it isnecessary to execute additional functions in the DC distributed systemsuch as an energy management strategy to maintain the energy storageelements within their charge limits or grid ancillary functions (loadshifting, grid support, etc). Frequently, these additional functions arecoordinated by the inverters connected to the AC utility grid but theyare still needed when operated independent from the AC utility or insystems with low power interconnection to the AC utility. Traditionally,these additional functions (SECONDARY FUNCTION) demand additionalcoordination and communication amongst the different componentsconnected to the DC bus.

Wired and wireless communication means are commonly used to achievepower and energy balance between the different elements at the expenseof extra cost and low flexibility. Intentionally changing the value ofthe DC bus voltage called voltage signaling has been used as a means ofcommunication amongst elements in the power management of a DCdistribution system. The present implementations and their limitationscan be divided in two groups:

-   -   Some implementations use a single element or small group of        elements to maintain the bus and intentionally change the        voltage so that discreet voltage levels in the DC bus indicate        changes in operating mode to the other elements connected to the        bus. The new operating mode shifts the responsibility to        maintain the DC bus voltage to a different element and forces a        quick reaction of all the elements in order to maintain        operation. Precise voltage measurements for all the components        connected to the bus are fundamental to the successful operation        of the system. Also, aging and tolerances in sensors degrade        highly the performance of the DC distributed system.    -   In other cases, DC bus voltage changes, resulting from changes        in the power balance, trigger reactions in multiple or all of        the elements to balance the power and maintain the DC bus within        a specified range of voltages. In this case, the system        stability is difficult to achieve and oscillations are common.        Furthermore, for these concepts it is not possible to        incorporate a secondary function, such as energy management, on        top of the power balancing.

In general, all of these proposals require major re-engineering whenadditional resources or loads are added to the DC microgrid or when anew installation with different ratings is implemented.

SUMMARY OF THE INVENTION

The invention describes a method for the operation and control of a DCmicrogrid system to achieve, in addition to DC bus voltage regulation, asecondary performance function such as energy management using only theDC bus voltage as means of communication. The proposed method uses oneelement on the microgrid as the brain of the microgrid operation (MAINELEMENT).

Because all the power elements connected to the DC distribution systemare connected to the DC bus, all of them will have access to a DC buslink voltage measurement (normally this voltage measurement is part ofthe power converters as it is needed for control and protection). TheDC-link voltage is maintained in a classical way by the MAIN ELEMENTexecuting a control algorithm like the one represented in FIG. 3. Inaddition, the MAIN ELEMENT has internal operating restrictions, such asits state of charge, and executes a local algorithm representing asecondary function to ensure its operation within those operatingrestrictions. The output of this algorithm is a change in DC voltage setpoint such that the average DC voltage is slightly drifted from thenominal value. Because the MAIN ELEMENT has the capability to maintainthe DC bus voltage, only a modification in its software is needed tointentionally vary the average DC-link voltage depending on theSECONDARY FUNCTION.

All the elements connected to the DC bus except the MAIN ELEMENT aremeasuring the average DC link voltage and reacting to it by changingtheir output in a manner such that the full effect is the fulfillment ofthe SECONDARY FUNCTION. All these elements are not aware of therequirements, limitations, or constraints of the SECONDARY FUNCTION butsimply follow the direction from the MAIN ELEMENT that has beencommunicated using the average DC link voltage. Note than only the MAINELEMENT is responsible for the fast regulation of the DC bus voltagewhile all the other elements respond to the much slower average voltage.This increases the stability of the system and enables self-calibration.The changes in the power generated or consumed by each element exceptthe main element with respect to the DC bus voltage has slow transientsresulting in smooth gradual transitions instead of discreet suddenchanges. As a result, the system stability is enhanced and thesuccessful operation is not affected by normal tolerances in the voltagesensors embedded in the different components.

According to one embodiment of the invention, a battery storage elementcoupled with a power converter represents the main element of thesystem. The main element or energy storage unit is commonly providedwith sensors to generate an estimation of the state of charge for theunit. The state of charge has to be maintained within limits to enhancethe performance, life expectancy or safety for the energy storage unit.Maintaining the state of charge within those limits represents thesecondary function. The intelligence embedded in the energy storage unituses the estimated state of charge to change the set point for the DCbus voltage. Since the energy storage unit is capable of regulating theDC bus voltage, the average voltage of the DC bus follows this setpoint.

According to another embodiment of the invention, an AC/DC converterexchanging power between the AC grid and the DC bus is the main elementof the installation. The main element is programmed or periodicallyreceives a loading pattern that depends on the cost of the electricityand time of the day and that represents the secondary function. TheAC/DC converter is then regulating the DC bus voltage while changing itsset point to produce a response that follows the loading pattern.

In another aspect of the invention, the multiple elements different fromthe main element are connected to the DC bus either directly or througha power converter. These elements may include solar PV generation,conventional generators, controllable loads, etc. Some or all of theseelements have the capability to measure the DC bus voltage and to limitthe power consumed or generated by them as a function of the measured DCbus voltage. These elements are then programmed with local functions tomodify their power and support the main element in maintaining the stateof charge within the desired limits.

Is another aspect of the invention that the local functions of theelements different from the main element are different from each otherand selected based on each element preferred operating conditions andoperational cost.

Is yet another aspect of the invention that the different elements canbe physically distant from each other and operate without anyinformation about the other elements while still cooperation to thefulfillment of the secondary function. In addition, the centralizedcontrol of the average DC bus voltage and the smooth response of thepower elements responding to the average DC bus voltage provide a trueplug-and-play functionality where power elements can be added or removedfrom the DC bus without the need for re-engineering or re-tuning of thesystem.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a microgrid based on a DCdistribution system where multiple elements are joined andinterconnected in a common DC bus.

FIG. 2 is a schematic representation of the present invention in generalterms where one element is shown as the main element executing theprimary function to regulate the voltage as well as the second functionthat is broadcasted using the DC bus voltage. The other power elementsreceive the broadcasted message by measuring the DC bus voltage and actaccordingly modifying their power

FIG. 3 is a block diagram for the regulator executing the DC bus voltagecontrol and of the algorithm producing a deviation of the DC bus voltageset point to fulfill a secondary function. Both of these functions arelocated in the main element.

FIG. 4 is a block diagram of the response of the elements connected tothe DC bus different from the main element to modify their power asfunction of the DC bus voltage.

FIG. 5 is a schematic representation of one embodiment of the inventionwhere the main element is associated with a battery storage unit.

FIG. 6 is a schematic representation of another embodiment of theinvention where the main element is associated with a bidirectionalDC/AC converter.

FIG. 7 is a full representation of a DC microgrid with the differentfunctions embedded in the different elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention presents a method to interconnect and control a DC powerdistribution system composed by multiple independent power elementsincluding power generators and power consumers. The method enablesachieving multiple levels of control using the DC bus voltage as theonly mean of communication amongst the power elements. The primary levelof control is the DC bus voltage regulation while the second level ofcontrol involves an independent variable that has slower dynamicresponse than the DC bus voltage. In other words, the changes in theindependent variable controlled by the second level of control areconsiderable slower than the changes in DC bus voltage.

A DC distribution system uses a common DC bus to join multiple powerelements and the continuous operation of the system depends onmaintaining the DC bus voltage regulated within the normal operatingrange. Nevertheless, the different power elements connected to the DCdistribution system have individual requirements to operate in theirmost efficient, stable and durable manner that are completelyindependent of the DC bus voltage regulation. Some DC distributionsystems use one element to execute the DC bus voltage regulation.However, for complex DC distribution systems including different typesof power elements and operating modes the DC bus voltage regulation mayneed to be shared between different power elements.

The proposed method represented in FIG. 2 uses one of the power elementsin the DC distribution system defined as the main element 100 to executethe primary level of control or DC bus voltage regulation 110 under allthe operating modes of the DC distribution system. The main elementensures that the average value of the DC bus voltage is approximatelyequal to a set point or reference value. Therefore, it has to respondquickly by changing its power when the measured voltage deviates fromthe set point.

In most practical distribution systems, the main element has alimitation of for how long it can continue regulating the DC bus voltagebefore degradation or unsafe conditions occur. For the embodiment shownin FIG. 5, an energy storage system is used as the main element. In thiscase, the state of charge 120 provides an indication of the capabilityfor the energy storage unit to continue regulating the DC bus voltage.To ensure the continuous operation of the DC distribution system overlong time, the state of charge should be maintained within a specificrange. The state of charge represents a secondary variable that isindependent of the power required to regulate the DC bus voltage butthat has to be controlled. In the proposed method, the main element alsoincorporates a secondary function 130, represented in FIG. 3, anddependent on the secondary variable 120 that results on a preferred mainelement power command 140 so that the main element maintains thesecondary variable within its design limits.

Previous art uses complex algorithms to divide the power command amongstall the elements connected to the DC distribution system and thencommunicates the result to each element. According to this invention,the main element converts the power command 140 to a DC bus voltageset-point deviation using a function 150 embedded in the main elementand the modified set-point is followed by the main element DC busvoltage regulation 110. Since the voltage set-point changes at a ratemuch slower than the dynamics of the DC bus voltage, the system willmaintain a regulated but slowly changing DC bus voltage. The powercommand is consequently broadcasted using the DC bus voltage 300 to allthe elements connected to the DC distribution system. The range ofvariation for the DC bus voltage set point should be limited within arange that do not result in any degradation in performance or incorrectoperation of any of the power elements.

All the power elements connected to the DC bus that are not the mainelement 200 and that will participate in the control method incorporatethe capability to measure the DC bus voltage 210. The measured DC busvoltage is then filtered 220 to ensure that these elements will onlyrespond to the slowly changing average DC voltage and not to transientsresulting from typical elements connected to the DC distribution system.The filtered DC bus voltage is used internally by each element tocalculate the power reference 230 for that element generation orconsumption. The power is regulated internally by the element using wellknow controllers 240. Because of the heavy filtered voltage used in thepower reference calculation and the slow changes of the average DC busvoltage regulated by the main element, the transients in power are slowand smooth providing a more stable system.

The algorithm to set the average DC-bus voltage as function of thesecondary variable may include linear, quadratic, integral, or derivateterms amongst others. It would be recognized for those skilled in theart that the secondary variable in addition to the state of charge oraverage power could be associated with energy, power, current,temperature or any other variable that can be regulated based onaccumulated energy or average power. The variable to be used depends onthe specific constrains for each application. According to thisinvention, the average DC bus voltage is continuously changed by theMAIN ELEMENT within a small range. Since the changes are smooth andprogressive, the system is robust against tolerances in the DC busvoltage measurement of the different components. In other words, if asmall change in the average DC bus voltage does not produce the expectedresponse, the average DC bus voltage is changed even more until thedesired response is achieved.

It should be understood that that the function on each of the powerelements different from the main element changing the generated orconsumed power as a function of the average DC bus voltage can use othervariables such as current, fuel injection, etc. that result in a netchange of average power. Furthermore, this function does not necessarilyhave to follow a linear relationship with the average DC bus voltage.Instead if may include other linear or non-linear terms depending on thespecific properties of each distributed resource. For example, in asystem combining solar with wind and/or Fuel Cells in a DC distributionsystem, it may be preferable that the operation of the fuel cell ismaintained close to maximum power in order to maximize the efficiencyand lifetime of the generator while the solar and wind can be rampedwithout performance penalty. In this embodiment, the fuel cell power canbe stepped down when the average DC-bus voltage reaches a higher valuewhile than the one used to ramp the solar and wind. In general, theaverage DC-bus voltage is variable according to a function preprogrammedin the MAIN ELMENT as in (1), and the power, current, energy, or otheradjusted variable from each power element in the DC distribution systemis decided based on the average DC-bus voltage using a pre-establishedequation as in (2)

Vdc _(Link) =f(P,E,I)   (1)

P,E,I=f(Vdc _(Link))   (2)

The limiting functions for the different generators and loads may bedetermined by cost, performance, or durability decisions and may differfrom one generator to another in the same DC distribution system.

FIG. 7 shows the generalization of the proposed method for a microgridbased on a DC bus. Alternative and conventional power generators andloads in a common DC-bus, operate in unison forming a microgridexecuting energy management in addition to power managements withoutclassical communication amongst the power elements. An energy storageresource is used to instantaneously balance the generation with the loadand maintaining the DC-bus voltage at the desired level. A controllerthat may or may not be part of the energy storage unit is responsiblefor keeping the state of charge for the energy storage within limits.The energy storage controller adjusts the average DC-bus voltage to getmore or less energy from the distributed resources and the distributedresources could have different power vs average DC-bus voltage (PvsV)functions depending on their particular operating cost and preferredoperating mode. The PvsV equations can automatically change during theday or during different seasons during the year to optimize theoperating cost of the micro-grid.

Although the example in FIG. 7 shows the loads as uncontrolled, someloads with lower criticality could also be programmed with PvsVequations or “shaved” based on the average DC-bus voltage so that theyshut down partially or totally if there is low generation in themicro-grid and the energy storage is reaching low levels of charge. Thiswill give full flexibility to controller to adjust the energy productionand energy consumption within the microgrid without the need forcommunication to ensure the stability and continuous operation of thesystem. As in the case of the generators, the loads equation as functionof the voltage could change depending on the time, season or otherexternal characteristics.

It should be understood that the invention is not limited in itsapplication to the examples and preferred embodiments presented herein.Variations in the main element, use of multiple main elements incoordination, diversity of power elements, can be foreseen as using thesame control concept presented in this invention. Furthermore, DCdistributions systems packaged as a single unit, where the physicaldistance amongst elements is small, or using multiple DC busses atdifferent voltages can also employ the concepts proposed in thisdiscussion and constitute alternative aspects of the present invention.

What is claimed is:
 1. A method to interconnect and control severalpower elements including conventional or alternative power generators,loads, and/or energy storage units in a DC distribution system, whereinthe method comprises: providing a common DC bus to interconnect thepower elements to the DC distribution system using power converters;implementing multiple levels of control to achieve the durable andstable operation of the DC distribution system; the primary level ofcontrol, executed by a primary function, has as a goal the regulation ofthe DC bus voltage; at least one secondary level of control with slowerdynamic response than the primary level of control, executed by asecondary function that is based on a second variable independent of theDC bus voltage regulation, such that the secondary level of control hasa goal to maintain the second variable within specified limits; using afirst group of one or more of the elements (main element) to execute theprimary function and automatically maintain the DC bus voltage followinga set point; intentionally changing the DC bus voltage set point withslow dynamics according to one of the secondary functions such that theaverage DC bus voltage regulated by the main element changes providingall the power elements connected to the DC bus other than the mainelement with voltage sensors so that they can measured the changes inaverage DC bus voltage; and using a local logic on each of the powerelements except the main element connected to the DC bus to modify theirpower generation or consumption as a result of changes in the measuredaverage DC bus voltage such that they contribute to the fulfillment ofthe secondary level of control.
 2. The method according to claim 1,wherein the main element modifies its DC bus voltage set point as acontinuous function of the second variable to provide for smoothtransitions in the system.
 3. The method according to claim 1 whereinthe power elements other than the main element change their powergeneration or consumption as a continuous function of the averagemeasured DC bus voltage to provide for smooth transitions in the system4. The method according to claim 2, wherein the continuous function usedto obtain the DC bus voltage set point can be changed hourly, daily orseasonally depending on climatic or economic indicators.
 5. The methodaccording to claim 3, wherein the continuous function for each elementother than the main element used to modify the power generation orconsumption depends on such element preferred operating conditions andis independent of any other element connected to the DC bus.
 6. Themethod according to claim 5, wherein the continuous function used tomodify the power generation or consumption can be changed hourly, dailyor seasonally depending on climatic or economic indicators.
 7. Themethod according to claim 1, wherein the main element is one or moreenergy storage devices connected to the DC bus through power convertersand one of the secondary levels of control is given by a set ofrequirements for energy or battery management that result in a secondaryfunction
 8. The method according to claim 1, wherein the main element isone or more grid connected DC/AC converters and the secondary level ofcontrol is given by a set of AC grid average power generation orconsumption constraints
 9. The method according to claim 1, wherein thedistribution system may run connected to the AC utility grid orindependent of it.
 10. The method according to claim 1, wherein thelogic executing the primary and secondary functions is embedded in themain element hardware components.
 11. The method according to claim 1,wherein the logic executing the primary and secondary functions islocated in a controller external to the main element.
 12. The methodaccording to claim 1, wherein the logic executing the change in powergenerated or consumed by the power elements other than the main elementis embedded in the respective power element hardware components.
 13. Themethod according to claim 1, wherein the logic executing the change inpower generated or consumed by the power elements other than the mainelement is located in a controller external to the main element.
 14. Themethod according to claim 1, wherein the distribution system controlledby the method represents a microgrid based on a DC bus.